Handle errors when gas_limit is calculated

[somoza]

When I have some guard on my contract like is_owner this throws an unhandled exception on the gas_limit calculation. I added a simple error tuple to be able to capture the event and don't kill my genserver while sending transactions.

Example of error I got before the change:

** (MatchError) no match of right hand side value: {:error, %{"code" => -32000, "data" => %{"hash" => nil, "message" => "revert", "programCounter" => 3727, "reason" => "Sender no autorizado.", "result" => "0x08c379a00000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000001553656e646572206e6f206175746f72697a61646f2e0000000000000000000000"}, "message" => "VM Exception while processing transaction: revert Sender no autorizado.", "name" => "RuntimeError", "stack" => "RuntimeError: VM Exception while processing transaction: revert Sender no autorizado.\n    at exactimate (/Applications/Ganache.app/Contents/Resources/static/node/node_modules/ganache/dist/node/1.js:2:182136)"}}
    (eth 0.6.7) lib/eth/query.ex:166: ETH.Query.estimate_gas!/2
    (eth 0.6.7) lib/eth/transaction/builder.ex:120: ETH.Transaction.Builder.build_params_from_map/1
    (eth 0.6.7) lib/eth/transaction/builder.ex:19: ETH.Transaction.Builder.build/1
    (eth 0.6.7) lib/eth/transaction.ex:28: ETH.Transaction.send_transaction/2
    ```